1. Field of the Invention
This invention relates generally to gain control circuits acting in general as amplifiers, and more particularly to gain control circuits of the differential amplifier type which have a pair of signal transmission paths operating differentially.
2. Description of the Prior Art
Gain control circuits of the differential amplifier type have been universally used for various kinds of electric apparatus, especially, in the form of an integrated circuit. In these gain control circuits, it is a basic structure that a differential amplifier which is formed by a pair of amplifying elements, such as transistors, and a current sink connected in common to the pair of amplifying elements is provided and the gain in amplification by the differential amplifier is controlled by means of changing the biasing condition of the amplifying element, changing the current value of the current sink or any other possible way. Generally, in practice, in order to achieve the gain control with increased efficiency and superior balance between both the amplifying elements, it has been proposed to provide a plurality of differential amplifiers in the form of the multistage type between an input terminal and an output terminal for signals to be gain controlled. For example, a differential amplifier of the first stage is provided to be supplied with an input signal and amplifying the same and an additional differential amplifier of the second stage is provided with a pair of input terminals connected to a pair of output terminals of the differential amplifier of the first stage, respectively, to further amplify the output of the differential amplifier of the first stage, and at least one of these differential amplifiers is gain controlled.
It is well known that in such a circuit of the multistage differential amplifier type as described above the gain control can be attained very effectively. The previously proposed gain control circuits, however, require a number of elements such as transistors and diodes for forming a gain controllable multistage differential amplifier and their circuit construction is relatively complicated. Furthermore, it is difficult for the previously proposed circuits to achieve a desirable balanced operation due to lack of uniformity in the characteristic of each element.